Collagen membranes for reverse osmosis desalination

ABSTRACT

A collagen membrane is employed as reverse osmosis membrane in desalination of water. The process is particularly applicable to desalination of brackish waters containing substantial amounts of divalent ions.

United States Patent 1 Feb. 22, 1972 Higley [54] COLLAGEN MEMBRANES FOR REVERSE OSMOSIS DESALINATION [72] Inventor: Willard S. Higley, 837 I-Iuerte Verde Road, Glendora, Calif. 91740 [22] Filed: Mar. 31, 1970 {21 Appl. No.: 24,337

[52] U.S.Cl ..210/23, 210/503 [51] Int. Cl ..B0ld 13/00 [58] Field ofSearch ...210/22, 23, 321, 500, 503; 264/41, 49

[56] References Cited UNITED STATES PATENTS 3,331,772 7/1967 Brownscombe et a1 ..210/22 X 3,228,876 1/1966 Mahon ..210/22 3,462,362 8/1969 Kollsman 210/321 X 3,413,219 11/1968 Kraus et al..... ...210/500 X 3,567,666 3/1971 Benz/er ..210/503 3,472,766 10/ 1969 Rosenbaum ..210/500 X 3,131,433 4/1904 Fexrani ..210/321 X 3,423,491 1/1969 McLain et a1 ..210/321 X OTHER PUBLICATIONS Primary ExaminerReuben Friedman Assistant Examiner-Richard Barnes AttorneyEmest S. Cohen and William S. Brown [5 7] ABSTRACT A collagen membrane is employed as reverse osmosis membrane in desalination of water. The process is particularly applicable to desalination of brackish waters containing substantial amounts of divalent ions.

5 Claims, No Drawings 1 COLLAGEN MEMBRANES FOR REVERSE OSMOSIS DESALINATION Reverse osmosis has attracted considerable interest in the field of purification of saline water. In this process, a pressure in excess of the osmotic pressure of the sale water feed solution is applied to the solution separated from purified water by a semipermeable membrane. Pure water is thereby caused to diffuse through the membrane, while the salt molecules or other impurities are retained by the membrane.

Efficiency'of the reverse osmosis process depends to a large extend on the nature of the semipermeable membrane and numerous types of membranes have been described in the prior art. Among the more effective of these has been the cellulose acetate membranes described, e.g., in U.S. Pat. Nos. 3,133,132; 3,133,137 and 3,439,074. These prior art membranes are cast from solutions comprising the membrane material and an organic solvent, with or without additional solution components such as water, swelling agents, etc. The membranes may be employed in the reverse osmosis process in the form of a free film or a film deposited on a porous support material.

it has now been found that a membrane consisting essentially of collagen is an effective semipermeable membrane for use in reverse osmosis processes, particularly where high flux and moderate salt retention are desired. These membranes are particularly well suited to desalination of brackish waters containing substantial amounts of divalent ions, e.g., magnesium or calcium. The membranes of the invention are cast from colloidal dispersions, as distinct from the prior art membranes 3 which, as discussed above, are cast from solutions. In addition, the membranes of the invention are essentially isotropic in structure, in contrast to the asymmetric cellulose acetate membranes of the prior art. The collagen membranes of the invention are, however, characterized by high absorption of water, resulting in high flux of product water through the membrane under conditions of reverse osmosis operation.

The membranes of the invention are preferably prepared from a microcrystalline form of collagen, i.e., collagen in which the average crystal size is less than about 1p. Colloidal casting dispersions of the collagen, preferably in concentrations of about 0.25 to 1.0 percent by weight, are prepared by dispersing the collagen in a mixture of water and an organic solvent such as methanol, ethanol, isopropanol or acetone.

Since the cast collagen films are adhesive in nature it may be desirable to employ casting surfaces that permit ready release of the dried films. Suitable surfaces of this type are wax-extended polyethylene sheeting and silicone release paper. The membrane may, however, be cast directly on a suitable support such as a filter material.

Cross-linking of the collagen films is usually desirable in order to provide sufficient wet strength. This may be achieved by either thermal or chemical means or a combination of the two. Drying the wet membrane overnight at about 3550 C.,

followed by heat treatment at a temperature of about ll to 150 C. for about 2 to 6 hours has been found to give good results. Addition of a cross-linking agent such as hexamethoxymethylmelamine to the casting solution in an amount of about 0.06 percent, and subsequent heat treatment of about ll0 to 150 C., also gives a film having good strength and desalination properties. Examples of other suitable cross-linking agents are glyoxal, quinone and chrome alum. Optimum treatment conditions for a given cross-linking agent are best determined experimentally.

The collagen membranes of the invention have the additional advantage that they may be deposited on porous substrates having an average pore size less than the average particle size of the dispersed colloidal collagen particle. Examples of suitable substrates are Metrical filters of about 2,000 A. average pore size. These filters consist of cellulose triacetate and are manufactured by Gelman Instrument Company, Ann

Arbor, Mich.

The following example will serve to more particularly illustrate the invention.

EXAMPLE 1 A casting dispersion was prepared by dispersion 0.64 wt. of 113 H, a microcrystalline collagen (crystal size lp.) in a Ill (wt./wt.) solution of water-methanol in a Waring Blender. Avitene H, developed by Food Machinery Corporation, Princeton, NJ. is prepared from bovine hide collagen by a process described in Journal of Applied Polymer Science, I l, 481, (1967). Collagen and water-methanol mixtures were charged to the blender, allowed to soak for'5 minutes,

wax-extended polyethylene sheeting taped to glass.

The film was dried overnight at 47 C. and then heat-treated at 150 C. for 6 hours to effect cross-linking. Three circles of the film were water-wet and mounted on No. 50 Whatman filter paper in a reverse-osmosis unit and tested at 800 p.s.i. using 10,000 p.p.m. magnesium sulfate feed solution. The 1.45 mil wet-thick film held up well with actual fluxes of 5.0 to 5.2 gfd. and 74.4 to 75.4 percent magnesium sulfate rejection. A limited life study was continued by allowing the film to stand in contact with the feed solution for 24 hours after the initial shutdown. it was then restarted, run for an addition 1.6 hr., and shut down for 72 hr. and then restarted'again. Over this period, the actual flux increased slightly to 5.8 gfd. and the magnesium sulfate retention fell to 69.3 percent, indicat- 40 ing only a slight deterioration of reverse-osmosis properties.

The calculated fluxes of the collagen membranes based on wet thickness of 2,500 A. ranged from 734 to 822 gfd. with magnesium sulfate rejections of 69.3 to 75.4 percent over the same time range. Cellulose acetate membranes of the same 4 wet thickness would give only 10 to 12 gfd. under similar conditions. Thus, the high fluxes demonstrated for collagen membranes make them unique for magnesium sulfate desalination.

EXAMPLE 2 Collagen films were cast on solvent-wet Metricel filters of 2,000 and 75 A. average pore size to give a membrane consisting of a collagen film bonded to the surface of the supporting filter. Collagen casting dispersions (0.64 percent microcrystalline collagen with 0.056 percent hexamethoxymethylmelamine cross-linking agent) in a l:l water-methanol mixture were cast on the solvent (1:1 water-methanol) filled Metricel filters followed by air drying and heat treatment at 150 0 C. for 6 hours. The composite membranes were tested in a reverse osmosis cell at 800 p.s.i. pressure with 1.0 percent sodium chloride feed solution. The details are presented in Table l.

The above composite membranes would be expected to give .much higher retentions using a brackish water feed solution containing a preponderance of covalent ions such as Ca and TABLE 1 Dried film Calculated flux thick- Filter Oper- Actual (2,500 A. thick) Salt Cast wet ness, pore ation flux, retention thickness, mils mils size, A. hours gf. Dry Wet T percent deposited on a porous support.

3. The process of claim 1 in which the collagen membrane is formed by subjecting a collagen film to a thermal or chemical treatment to effect cross-linking.

4. The process of claim 3 in which the cross-linking is effected by means of heat treatment at a temperature of about to C.

5. The process of claim 4 in which chemical cross-linking is effected by inclusion ofa cross-linking agent in the dispersion used to form the collagen film. 

2. The process of claim 1 in which the collagen membrane is deposited on a porous support.
 3. The process of claim 1 in which the collagen membrane is formed by subjecting a collagen film to a thermal or chemical treatment to effect cross-linking.
 4. The process of claim 3 in which the cross-linking is effected by means of heat treatment at a temperature of about 110* to 150* C.
 5. The process of claim 4 in which chemical cross-linking is effected by inclusion of a cross-linking agent in the dispersion used to form the collagen film. 